1. Field of the Invention
The present invention relates to a functional coating for reducing friction, and more particularly to a urethane-resin based functional coating which is to be applied to glass sliding portions of a weatherstrip such as a glass run of an automobile, for reducing friction between an automotive window glass and the glass sliding portions of the weatherstrip.
2. Description of the Prior Art
Hitherto, there have been various proposals to coat glass sliding portions of a glass run of an automobile with coatings for the purpose of reducing friction between an automotive window glass and the glass sliding portions of the glass run. A glass run is usually made of rubber so that the glass run is vulcanized for improving its characteristics.
For example, JP-A-1-133711 discloses a mouthpiece structure of an extruder of glass run, whereby glass sliding portions of the extruded glass run are coated with a coating liquid, immediately after the extrusion of glass run.
U.S. Pat. No. 5,326,592 corresponding to JP-A-5-131519 also discloses a process and apparatus for coating selected surface areas of extruded substrates with decorative or functional coatings, immediately after the extrusion. The functional coatings of this publication may include various polymers such as polyurethanes for high abrasion resistance, polyesters for weatherability, and silicones and TEFLON (a trade name) polymers for low friction resistance.
JP-A-3-161329 discloses a method of coating glass sliding portions of an extruded glass run with a solvent-free urethane coating, immediately after the extrusion. For reducing friction, this urethane coating may contain a liquid-type agent such as a silicone oil (dimethyl polysiloxane) or a solid-type agent such as TEFLON resin powder, molybdenum disulfide, boron nitride, tungsten disulfide or fluorinated graphite. The extrudates which have been coated with coatings by the methods of all the above publications are vulcanized later.
JP-A-57-170314 discloses a glass run having glass sliding portions on which fine particles (diameter: 5-500 .mu.m) are adhered for reducing friction.
JP-B-1-24643 discloses a coating to be applied to glass sliding portions of a glass run. This coating may be a solventthinned two-package type urethane-resin coating and contains first large-diameter particles (diameter: 10-30 .mu.m) such as particles of talc and second small-diameter particles (diameter: 2-10 .mu.m) such as particles of tetrafluoroethylene (the most preferable example), molybdenum disulfide tungsten or nylon. The second small-diameter particles surround each first large-diameter particle and adsorb thereto. Thus, third large-diameter particles each having one first large-diameter particle surrounded by the second small-diameter particles are formed, as illustrated in this publication.
As is disclosed in JP-B-1-24643, when a solvent-thinned urethane-resin coating containing fine particles is applied to glass sliding portions of a glass run by spraying, the average thickness of a coating film will be about 20 .mu.m and it is difficult to substantially increase the coating film thickness. Furthermore, the film thickness tends to become uneven if spraying is taken. Thus, in case that the film thickness is less than the diameter of the fine particles (for example, the third large-diameter particles disclosed in JP-B-1-24643), part of the fine particles will be exposed. With this, the film surface becomes minutely rough, and thus friction between a window glass and the glass run is efficiently reduced. However, in case that the film thickness is greater than the diameter of the fine particles, the fine particles are less exposed or not exposed at all. With this, the film surface becomes relatively flat, and thus the friction is not efficiently reduced. When the fine particles of the film are partly exposed and partly not exposed on a glass run, this deteriorates the external appearance of the glass run, too. Furthermore, as the density of the fine particles exposed on a glass run increases, the sliding noise of the window glass increases. Irrespective of coating method, it is unavoidable to have certain variations in thickness of the coating film. Therefore, it is considered to substantially increase the diameter of the fine particles or substantially increase the fine particle content of the coating so as to expose the fine particles above the coating film surface and to reduce the friction. However, if the diameter of the fine particles is increased too much, the orifice of an extruder tends to be clogged with the fine particles. Furthermore, if the fine particle content of the coating is increased too much, viscosity of the coating tends to become too high, thereby impeding the coating process.